MR. J. T. HOTBLACK ON* STONES ON MUNDESLEY BEACH. 11 
theories have been propounded for the purpose of explaining the 
origin of agate-nodules in the cavities of rock. The cavities 
themselves are supposed to result from the imprisonment ot 
gas-bubbles whilst the rock was in a molten condition. 1 he 
nodules have been deemed to result from the crystallisation of 
silica whilst in solution, or from the driving off of silica from the 
surrounding rock in a state of vapour, whence it slowly solidified. ” 
I should rather have said that the bubbles were of silica in 
a state of vapour, which in cooling and solidifying formed crystals 
in the centre of such geodes as were sufficiently large. 
The same author says : “ Agate, onyx, and chalcedony are 
produced almost exclusively in trap.” lie should have said 
agate, onyx, and the other forms of chalcedony, for with Bristow 
1 contend that carnelian, sard, onyx, sardonyx, agate, &c., are all 
forms of chalcedony. 
Thompson says: “Chalcedony comprises carnelian, bloodstone, 
onyx, sardonyx, mochastone, agate, &c.,” and it is not at all difficult 
to say what the &c., comprises. 
How has all this confusion arisen 1 It is, 1 take it, because — 
quartz, chalcedony, and flint are each of them almost of exactly 
the same chemical composition, and they have the same cleavage, 
lustre, hardness, and specific gravity, and yet the two former are, 
as I believe, always of igneous origin, while flint we know was 
formed in quite a different way. There was a very able paper 
on the Formation of Flints, read by Professor W. J. Sollas, F.R.S., 
before the last meeting of the British Association. No, I think 
it a mistake to call flint or stalactites of silica either quartz or 
chalcedony, the former no doubt have sometimes a beauty of their 
own, but they could never have been used either for the engraving 
of seal and intaglios, or the carving of cameos, for which chalcedony, 
from the earliest ages, has been recognised as peculiarly adapted. 
There is something in the toughness, quite apart from the hardness, 
of the whole chalcedony family, which makes every sort most 
suitable for the carver, graver, or engraver. Sard, carnelian, agate, 
onyx, sardonyx, &c., are all equally good. I could give any 
number of quotations from ancient writers extolling their superiority 
over all other stones for the making of seals, cameos, &c. To-day 
they are almost exclusively used for the making of seals even in 
the wholesale trade. Were you to take a flint (however much it 
